This invention relates generally to microelectromechanical systems (MEMS), and more particularly the invention relates to torsional and linear vertical comb MEMS actuators and to the microfabrication of the actuator.
Electrostatic comb-drive actuators are used in numerous MEMS applications where they have demonstrated their capability for extended ranges-of-movement, stable and reliable operation, and design flexibility in different frequency ranges. In particular, many optical MEMS applications employ comb-drive actuators for torsional and linear motions. For example, lateral comb-drives and mechanical hinges or linkages made of polysilicon or single-crystal silicon have been demonstrated to make torsional actuators. From these applications, linkage- and hinge-designs have been identified as sources for reliability problems and for limitations on maximum frequencies of operation. One design for torsional actuators uses vertically aligned comb-drives to achieve both higher frequencies and larger scan angles than those characterizing the planar polysilicon structures. Other vertically aligned comb-drives have employed polysilicon on SOI and single-crystal silicon fabricated using wafer bonding.
FIG. 10 is a plan view of an Angular Vertical Comb-drive (AVC) actuator including a device structure 10 (micromirror for example) which is supported by torsion springs 12 on a support layer 14. Typically, layer 14 is a silicon on insulator (SOI) layer and device structure 10 and torsion springs 12 are fabricated therefrom by conventional photoresist masking and etching techniques. Comb-drive actuators 16 comprising stationary comb fingers on support layer 14 and interdigitated moveable comb fingers on device structure 10 rotatably move device structure 10 on torsion springs 12 in response to electrical drive signals applied to the comb fingers on device structure 10 through electrical contacts 18.
The basic fabrication processes for AVCs and SVCs (Staggered Vertical Comb-drive) are first to define the stationary and movable comb structures on the same level of silicon layer, and to deflect either the stationary comb structure by the residual stress induced by a metal layer, or else the movable comb structure using the surface-tension force that arises as a result of the reflow of a patterned-photoresist layer. A limitation of the residual-stress method of fabrication is that the structure must be sufficiently flexible so that it undergoes appreciable deformation. Designs that meet this requirement are generally limited in their ultimate operating frequencies to a few hundreds Hertz, Hz. The challenging problems for the reflow of polymer hinges have been the control and reliability of the polymer material.
To improve the performance of torsion-bar micro-actuators, AVC-type torsional actuators that are composed of all-single-crystal-silicon structures have been fabricated using controlled-plastic-deformation in silicon that is annealed at elevated temperatures. In recent years, polysilicon and single-crystal silicon membranes were demonstrated to be plastically deformed forming hemispherical domed structures as a result of the pressures of heated gases trapped in a cavity. Plastically deformed polysilicon structures have been used in a self-assembled MEMS process.
The present invention is directed to a vertical comb type actuator with novel torsion springs and to a novel method of fabricating the AVC and SVC type actuator including thermal processing of the torsion springs to cause a permanent deflection of the device structure relative to the support structure.